1. Field of the Invention
The present invention relates a light scanning apparatus and an image forming apparatus using this light scanning apparatus, and is suited to an image forming apparatus, such as a laser beam printer and a digital copying machine that have an electrophotographic process, and a multi-function printer.
2. Description of the Related Art
According to the prior art, in a light scanning apparatus, such as a laser beam printer (LBP), light beams optically modulated and emitted from a light source means are periodically deflected by a light deflector (a polygon mirror, etc.) corresponding to image signals. The deflected light beams are converged in a spot-like shape on a photosensitive drum through a light scanning optical system having a characteristic fθ. An image is recorded by scanning with the converged light beam.
FIG. 16 is a principal schematic view of the conventional light scanning apparatus.
Divergent light beams emitted from a light source means 81 are substantially collimated by a collimator lens 83. The collimated light beams are restricted by a stop 82 and are made to be incident on a cylindrical lens 84. The cylindrical lens 84 has a predetermined refracting power only in a subscanning direction. In the collimated light beams incident on the cylindrical lens 84, the light beams within a main scanning section emerge therefrom in an as-is state. Further, within a subscanning section, the light beams are converged and form an image substantially as a line image on a deflection surface 85a of a deflection means 85 constructed of a rotary polygon mirror.
Then, the light beams deflected by the deflection surface 85a of the deflection means 85 are guided to a photosensitive drum surface 87 via an imaging optical system 86. Subsequently, the photosensitive drum surface 87 is optically scanned in a direction of an arrowhead B by rotating the deflection means 85 in a direction of an arrowhead A.
In the light scanning apparatus described above, a BD sensor 89 defined as a photo detector is provided for adjusting timing of starting the image formation on the photosensitive drum surface 87 before scanning the photosensitive drum surface 87 by the light spots.
This BD sensor 89 receives BD light beams as part of the light beams reflected in deflection by the light deflector 85. The BD light beam connotes a light beam when scanning an area, excluding an image formation area, before scanning the image formation area on the photosensitive drum surface 87.
The BD light beam is reflected by a BD mirror 88 and becomes, after being converged by a BD lens (not shown), incident upon the BD sensor 89.
Then, a BD signal is detected from an output signal of the BD sensor 89, and the detected BD signal is inputted to an image processing unit 91. The inputted BD signal is taken in synchronism with an image clock for scanning the image. Then, the timing of starting of recording the image is controlled.
The image signal outputted from the image processing unit 91 is outputted to a semiconductor laser operating unit 92 in accordance with the image clock at an image write start timing. Further, information given from photo-diodes disposed in the vicinity of the laser within the semiconductor laser 81 is detected, and APC (Automatic Power Control) is conducted, so that an emission power of the semiconductor laser 81 becomes a standard light intensity from this information.
An effect given by the APC is, however, to control the light intensity from the semiconductor laser 81 to a predetermined value. What is actually required, however, is to control the light intensity on the photosensitive drum surface.
FIG. 17 shows a transmittance characteristic of a general type of glass material. As shown in FIG. 17, generally, the transmittance is substantially constant at 90% or larger up to the vicinity of a visible wavelength of 450 nm from an infrared region. The glass, however, generally absorbs the light in an ultraviolet region, and hence, the wavelength of the transmissible light has a lower limit (an absorption end). The transmittance abruptly decreases in a region where the wavelength is shorter than a wavelength on the order of 450 nm.
By the way, a much higher printing accuracy has been demanded of the apparatus in recent years. There have recently been proposed a variety of light scanning apparatus using a light source that irradiates the light of which the wavelength is shorter than the wavelength of 450 nm, such as a Blue laser (blue-violet semiconductor laser) (see, for example, Japanese Patent Application Laid-Open No. 2002-277803).
A contrivance of Japanese Patent Application Laid-Open No. 2002-277803 is that high color saturation of the print is attained in a way that decreases a size of the light spot formed on the scanned surface by the light scanning apparatus, which involves the use of a light source irradiating the light, of which the wavelength is equal to or shorter than the wavelength of 450 nm.
In the region of the wavelength equal to or less than 450 nm, as described above, a change in the transmittance of the glass material with respect to the change in the wavelength of the semiconductor laser is larger in the conventional infrared region.
Accordingly, when an oscillation wavelength of the laser changes due to a change in temperature, even if the light intensity of the semiconductor laser is kept constant by the APC operation, the light intensity on the photosensitive drum surface does not become constant, due to the change in the transmittance of each optical element. As a result, such a problem arises that reproducibility of the image cannot be preferably maintained.